The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a self-servo writing method and system for a storage device.
Disk drives for computer systems comprise a disk for storing data and a head actuated radially over the disk for writing data to and reading data from the disk. To effectuate the radial positioning of the head over the disk, the head is connected to the distal end of an arm that is rotated about a pivot by a rotary actuator (e.g., a voice coil motor (VCM)). The disk is typically divided into a number of concentric, radially spaced tracks, where each track is divided into a number of data sectors. The disk is typically accessed a data sector at a time by positioning the head over the track that comprises the target data sector. As the disk spins, the head writes transitions (e.g., magnetic transitions) in the data sector to record data, and during read operations senses the transitions to recover the recorded data.
Accurate reproduction of the recorded data requires the head to be positioned very close to the centerline of the target data sector during both write and read operations. Thus, accessing a target data sector involves positioning or “seeking” the head to the target track, and then maintaining centerline “tracking” while data is written to or read from the disk. A closed loop servo system typically performs the seeking and tracking operations by controlling the rotary actuator in response to position information generated from the head.
A conventional technique for generating the head position control information is to record servo information in servo sectors disbursed circumferentially about the disk, “embedded” with the data sectors. FIG. 1 is an illustration of a disk showing sector servo data. A disk 100 comprises a number of concentric tracks 105 and a number of embedded servo sectors 110. Typically, each servo sector 110 is written on the surface of disk 100 as a radially-extending set of spokes or “wedges.” Each servo sector 110 comprises a preamble 115, a sync mark 120, servo data 125, and servo bursts 130. The preamble 115 comprises a periodic pattern that allows proper gain adjustment and timing synchronization of the read signal, and the sync mark 120 comprises a special pattern for symbol synchronizing to the servo data 125. The servo data 125 comprises identification information, such as sector identification data and a track address. The servo control system reads the track address during seeks to derive a coarse position for the head with respect to the target track. The track addresses are recorded using a phase-coherent Gray code so that the track addresses can be accurately detected when the head is flying between tracks. The servo bursts 130 in the servo sectors 110 comprise groups of consecutive transitions (e.g., A, B, C and D bursts) that are recorded at precise intervals and offsets with respect to the track centerline. Fine head position control information is derived from the servo bursts 130 for use in centerline tracking while writing data to and reading data from the target track.
The embedded servo sectors 110 are written to the disk 100 as part of the manufacturing process. Conventionally, an external servo writer can be employed that writes the embedded servo sectors 110 to the disks by processing each head disk assembly (HDA) in an assembly line fashion. The external servo writers employ very precise head positioning mechanics, such as a laser interferometer, for positioning the head at precise radial locations with respect to previously servo-written tracks so as to achieve very high track densities.
There are certain drawbacks associated with using external servo writers to write the embedded servo sectors 110 during manufacturing. For example, the HDA is typically exposed to the environment through apertures that allow access to the disk drive's actuator arm and the insertion of a clock head that requires the servo writing procedure to take place in a clean room. Furthermore, the manufacturing throughput is limited by the number of servo writers available, and the cost of each servo writer and clean room can become very expensive to duplicate.
Attempts to overcome these drawbacks include a “self-servo writing” technique in which components internal to the disk drive are employed to perform the servo writing function. Self-servo writing does not require a clean room. because the embedded servo sectors are written by the disk drive after the HDA has been sealed. In addition, self-servo writing can be carried out autonomously within each disk drive, thereby obviating the expensive external servo writer stations.
The self-servo write (SSW) process involves several steps. A clock synchronous to the disk rotation is generated by reading some timing marks from a reference signal on the disk and adjusting a clock generator in such a way as to lock the phase of the output clock to the timing marks. The clock is used for self-servo writing and will be referred to as SSWCLK. A wedge modulo counter is set up with a modulo count that equals the expected wedge-to-wedge distance (e.g., a predetermined target distance). The wedge modulo counter runs on the SSWCLK. With the SSWCLK locked to the disk, the wedge modulo counter is also counting synchronously relative to the disk. Thus, a trigger point can be set on the wedge modulo counter to activate a servo write sequencer. When the wedge modulo counter value reaches the trigger value, the servo write sequencer writes a new wedge of servo data on the disk.
For SSW, a complete set of wedges of servo data may already exist on the disk. However, a new set of servo wedge bands may be need to be replicated in between the original servo wedges. For example, the original servo wedges may have been written with such a low resolution that they cannot be used directly for normal disk drive servo operation. A new set of higher-frequency and higher-resolution wedges would need to be created using the low-frequency version as the reference. Alternatively, the original servo wedges may be of high frequency and high resolution, but suffer from some large, repeatable run-outs. The run-outs can degrade the performance of the drive. Repair work can be performed by re-generating a new set of servo wedges free from the large run-out problems.